In conventional computed tomography (CT) planar sections of a body are diagnosed by irradiating a substantial area of such planar sections by a source of penetrating radiation, detecting the attenuated beam passing through the patient, and processing the data accumulated by the detector. The data processing techniques used in conventional CT require the entire planar section to be scanned in order to reconstruct any part of it by traditional techniques utilizing back projection and convolution. Very often, however, there is only a relatively small area of interest in any particular cross section of the body, for example, in examining the spine. The advantage of using a small scan circle in such cases is the reduction of radiation dosage to the patient since all of the radiation beams pass through the smaller scan circle and better optical resolution in the area of interest. The latter is true because typically if a large scan circle is used to image a whole patient section, the resolution in the area of interest will be less than the full resolution capability of the scanner.
There are known techniques for preprocessing an incomplete projection obtained from a small scan circle which permit conventional CT algorithms to be applied to reconstruct images of the limited region of interest.
One projection completion method has been suggested by R. M. Lewitt, "Processing of Incomplete Measurement Data In Computed Tomography", Technical Report No. MIPG22, State University of New York at Buffalo, February, 1979, which involves the use of known boundaries of the object being scanned. This technique obtains a projection of a circle by performing a curve fit to the end of the truncated projection and to the previously determined baseline location of the edge of the object. This technique requires a method for measuring and digitizing the boundary locations of the edge of the object (outside the scan circle) as the scan is being performed. Though non-ionizing methods exist for defining the boundary, the method is dependent upon making measurements outside the limited region of interest defined by the small scan circle and hence involves additional expense and complexity.
Another projection completion method is that of coarse/fine sampling of the projections as suggested by O. Nalcioglu et al. in "Limited Field Of View Reconstruction In Computerized Tomography" NS-26 IEEE Transactions on Nuclear Science 546-51, 1979, and reported by R. Ethier of the Montreal Neurological Hospital, at 3 Journal of Computer Assisted Tomography 433-38, 1979. This technique involves performing high density sampling of the limited area of interest and coarse sampling of the surrounding tissue region to obtain the profile necessary to complete the projection. Presumably, a filter is inserted at the X-ray source to lower the dosage to the surrounding tissue. This requires a modification to the hardware configuration that performs the sampling process. Also, calibration must be done to obtain the profiles necessary to compensate for the attenuating filter at the source. In contrast, the preferred embodiment requires minimum modifications to a scanner.